This invention relates generally to an analog to digital pressure transducer system which is to be used as an altimeter/barometer in instruments such as a wristwatch, where only a low voltage power supply is available to drive the transducer. More particularly this invention relates to an improvement in pressure transducer systems of the type which use a dual-slope integrating analog to digital converter, or a derivative converter thereof.
It is commonly known that semiconductor piezoresistive strain-gauge bridges used in pressure transducer systems have a high positive temperature coefficient of resistance and correspondingly exhibit a high negative temperature coefficient of full-scale span. Thus at any given pressure, variations in ambient temperature will cause errors to be introduced in the output of the strain gauge bridge circuit, which if left uncompensated, will cause errors to be conducted in the output of the analog to digital transducer system itself, giving inaccurate readings as to the input pressure across the strain gauge bridge circuit's diaphragm.
Many methods of temperature compensation have been devised to compensate for such errors. However each of these methods requires a large voltage "overhead" for compensation and/or complex circuitry, both of which are undesirable in instruments of limited physical space and which are driven by a low voltage supply. Thus in order to implement an altimeter/barometer in a wrist instrument, where the device is to be operated from two silver oxide cells or from a single three-volt lithium cell, only a minimal voltage overhead is available for temperature compensation of the strain gauge bridge circuit output. Furthermore any requirement to regulate the battery voltage is undesirable because of the voltage required for regulation and because of the increased circuit complexity which results. Thus it would be desirable to provide a pressure transducer apparatus for a low voltage wrist instrument which requires minimal voltage to compensate for temperature induced errors and eliminates the necessity for voltage regulation.
As stated previously, many methods of temperature compensation are commonly known in the art. One common method of temperature compensation is to drive the strain gauge bridge circuit with a constant voltage source and to provide a resistor in series with the bridge input equal to approximately 3.6 times the strain gauge bridge resistance (R.sub.x), "Temperature Compensation, Calibration and Applications of Motorola's X-ducer Pressure Sensor," Harold Nagal, Motorola Semiconductor Application Note (AN922), p. 2 (1985); also. U.S. Pat. No. 4,510,813-Kanazawa. This method is unacceptable for low-voltage operation because of the large voltage drop that occurs across this series resistor. Another method of temperature compensation is to drive the strain gauge bridge with a constant - current source with a resistor, having a value of approximately 3.6 R.sub.x and connected across the bridge inputs; See. e.g. Motorola Semiconductor Application Note (AN922) at p.2. While this method does not necessitate as large a voltage drop, unwanted additional circuitry in the form of a series regulator and sense register are required in order to generate the constant current.
Additional methods of temperature compensation such as the use of networks consisting of resistors and thermistors or diodes are also undesirable since these temperature-dependent components have the disadvantage of causing an error in transducer output if they are not kept at the same temperature as the transducer. U.S. Pat. Nos. 4,788,521 - Johnson and 4,622,856 - Binder et. al. are examples of inventions which teach compensation for this effect through the design parameters of the passive piezoresistive elements themselves.
Still other methods make use of analog to digital converters as does this invention in order to compensate for errors introduced as a result of ambient temperature variations. Examples are seen in U.S. Pat. Nos. 4,192,055 -Kurtz, 4,715,003 - Keller et. al., and 4,765,188 - Krechmery et. al. Each however entails complex digital circuitry in which either the temperature-related strain gauge bridge circuit output voltage or the temperature of the piezoresistive strain gauge itself is digitized by the analog to digital converter and is then corrected by way of a programmable read-only memory (PROM) and a digital to analog converter, or directly corrected through the use of "lookup" table stored in a memory. As stated previously such complex circuitry is undesirable where the operation of the transducer is to occur in an instrument of limited physical space, such as a wristwatch.
Accordingly, one object of the present invention is to provide an improved circuit for an analog to digital pressure transducer which can be used as an altimeter/barometer in a wrist instrument to give a highly accurate output yet use very little voltage to drive the transducer.
Another object of the invention is to provide an improved circuit for an analog to digital pressure transducer which requires very little voltage to compensate for errors introduced into the strain gauge bridge circuit output as a result of ambient temperature effects.
Yet another object of the invention is to provide an improved analog to digital pressure transducer which eliminates the need for a stable voltage reference for the analog to digital converter; thus providing analog to digital conversion which requires less supply voltage and less complex circuitry.